Semiconductor devices and ICs are generally contained in semiconductor packages comprising a protective coating or encapsulant to prevent damage during handling and assembly of the components during shipping and when mounting the components on printed circuit boards. For cost reasons, the encapsulant is preferably made of plastic. In a liquid state, the plastic “mold compound” is injected into a mold chamber at an elevated temperature surrounding the device and its interconnections before cooling and curing into a solid plastic. Such packages are commonly referred to as injection molded using a method known as “transfer molding”.
Interconnection to the device is performed through a metallic leadframe, generally of copper, conducting electrical current and heat from the semiconductor device or “die” into the printed circuit board and its surroundings. Connections between the die and the leadframe generally comprise conductive or insulating epoxy to mount the die onto the leadframe's “die pad”, and metallic bond wires, typically gold, copper, or aluminum, to connect the die's surface connections to the leadframe. Alternatively, solder balls, gold bumps, or copper pillars may be used to attach the topside connections of die directly onto the leadframe.
While the metallic leadframe acts as an electrical and thermal conductor in the finished product, during manufacturing the leadframe temporarily holds the device elements together until the plastic hardens. After plastic curing, the packaged die is separated or “singulated” from other packages also formed on the same leadframe by mechanical sawing or by mechanical punching. The saw or punch cuts through the metal leadframe and in some instances through the hardened plastic too.
In “leaded” semiconductor packages, i.e. packages where the metallic leads or “pins” protrude beyond the plastic, the leads are then bent using mechanical forming to set them into their final shape. In other instances the metallic contacts to the semiconductor occur through conductors only accessible on the underside of the package. Such devices are known as “leadless” packages. Regardless of leaded or leadless construction, after manufacturing, finished devices are packed into tape and reels ready for assembly onto customers' printed circuit boards (PCBs).